The basis of chromatography is the separation of components of a sample owing to their differences in solubility or in adsorption in a stationary bed of a material (either liquid or solid). When the sample (moving phase) is a gas, the technique is termed either gas-solid or gas-liquid chromatography, depending on whether the stationary phase is a solid or a liquid. In gas chromatography, a sample is introduced into a carrier gas as a vapor which flows through a chromatographic system. Upon separation by the stationary phase, the sample components travel through the system at different speeds thereby entering a detecting device, attached to the system, at different times. As a result, individual components that are present in the sample may be identified by the detecting device.
A chromatograph such as a gas chromatograph, sometimes hereinafter GC, is an analytical instrument which can separate a gaseous mixture into its various constituent parts. A detecting device such as a mass spectrometer, sometimes hereinafter MS, is an analytical instrument which can qualitatively and quantitatively analyze a gaseous sample to determine its molecular structure. Both gas chromatographs and mass spectrometers have been around for a relatively long time. It has long been recognized that a powerful analytical tool could be obtained by coupling these two instruments. However, combination GC/MS instruments are relatively recent innovations, and continuing research and development is directed towards improving the interface between the gas chromatograph and the mass spectrometer portions of GC/MS instruments.
A typical GC/MS interface includes a tubular transfer line having one end coupled to the output of the gas chromatograph and having its other end extending into a vacuum chamber of the mass spectrometer. An ion source of the mass spectrometer is used to ionize the effluent from the transfer line, and a mass filter of the mass spectrometer is used to filter the ionized components of the gas according to mass. An ion detector within the vacuum chamber of the mass spectrometer detects ions filtered through the mass filter. Finally, a recorded output signal of the detector is studied to determine the chemical structure of the gas sample.
However, gas chromatographs generally operate at atmospheric pressure while mass spectrometers operate at greatly reduced pressures, generally at about 1.times.10.sup.-5 Torr. To balance such significant differences in pressures between the two devices, the GC/MS interface must provide some means to reduce the pressure of a sample gas leaving the gas chromatograph prior to its introduction into the mass spectrometer. Furthermore, since gas chromatographs operate by sweeping small amounts of sample gas through the GC column, at high volumetric rates of the carrier gas, some means must be found to enrich the concentration of the sample gas relative to the carrier gas before the gas mixture reaches the mass spectrometer. Failure to execute the enrichment step reduces the sensitivity of the mass spectrometer.
Since the gas chromatograph separates the various components of the sample gas or a sample material, the composition of the gas leaving the chromatograph varies with time. Because of the continually changing composition of the gas effluent, any mass spectrometer designed for use with a gas chromatograph must be capable of sweeping rapidly across the mass spectrum, for a swift analysis of the changing composition of the GC effluent.
A wide variety of approaches for interfacing gas chromatographs to mass spectrometers have been tried. The most common approach is a direct connection between the two by means of a capillary tube. The advantage of this type interface is its mechanical stability and the consequent ease with which it can be handled, especially when the chromatographic columns are not changed frequently. There are, however, a wide variety of disadvantages to this method. The major disadvantage of such a capillary restriction is that the sample material, including the solvent, elutes directly into the mass spectrometer source, thereby affecting the sensitivity of the MS. Additionally changing of the GC columns is a laborious and time consuming process as the mass spectrometer vacuum system has to be vented with each GC column change. Another drawback is that all of the gas effluent coming from the gas chromatograph is delivered to the mass spectrometer, thereby potentially overloading the mass spectrometer vacuum system. Thus the mass spectrometers normally designed to accept samples only in nanogram quantities, can be exposed to sample quantities in excess of milligrams. Such an extreme exposure to large amounts of elements causes contamination of the ion source, analyzer and vacuum system of the mass spectrometer, thereby increasing its maintenance cost and reducing the life of filaments used in the variable energy ion source of mass spectrometer.
A direct coupling of the GC to the MS also has an effect on the efficiency of the gas chromatographic separation. By directly coupling the GC to the MS the high vacuum of the mass spectrometer affects the GC column flow rates which results in shifting the GC retention times.
Another common alternative is the so called open-split interface. In its simplest form, one end of an interfacing capillary tube usually made of fused silica is used to provide a flow restriction into the mass spectrometer's GC inlet. The other end of the interfacing capillary tube is telescoped into or placed near the outlet of the GC capillary column. By adjusting the length and the inside diameter of the interfacing capillary tube, a natural vacuum induced flow is maintained to the MS without any permanent or elaborate physical connections between the GC and the interfacing capillary tube. Such an open-split interface has several advantages. The GC column is exposed to atmospheric pressure at its outlet, because it is not sealably connected to the MS. As a result the GC column is not affected by the low pressure of the MS. Secondly, when the aforementioned capillary tube is used as a restrictor, it has been shown that there is virtually no degradation of the chromatographic resolution. Thirdly, if the interfacing capillary tube is held at a constant temperature, by jacketing it in a chamber maintained at a constant temperature, the pressure to which the ion source of the MS is exposed, stays constant even if the GC oven temperature is profiled. Finally, if an accidental breakage of the GC column occurs, such an incident has no effect on the integrity of the MS vacuum.
Even though the aforementioned open-split capillary tube interface overcomes many of the problems associated with the GC/MS interface, it is woefully inadequate in addressing the major problem of solvent diversion which has major impact on the sensitivity and the functionality of the MS. The solvent diversion problem has been partially addressed by enclosing the open-split capillary interface in an evacuated enclosure or an enclosure flooded with an inert fluid, such as helium. A large volume of the inert gas is swept through the interface at the start of the process. Substantially large inert gas flow rates are necessary to flush away the solvent prior to its entry into the MS ion source. However, due to small internal volume of the interface, such large inert gas flow rates result in increasing the pressure within the interface to above the atmospheric pressure, thus increasing the GC retention times and the ion source pressure within the MS. As a result the ion source conditions within the MS may be adversely affected.
The gas chromatographs are also used in multiple stages to separate constituents of a component of the sample that may not be separated by a single stage GC column. Such a technique of separation, called multidimensional gas chromatography, utilizes more than one chromatographic column. Such high resolution multidimensional gas chromatographs (hereinafter MGC) generally have GC columns with different liquid phases. When a desired component of the sample is at a "peak" in the first stage it is switched to the second column for further separation of the component into its individual constituents.
The major problem faced by the multidimensional chromatography is a lack of a reliable switching scheme that operates at elevated temperatures without affecting the degree of resolution. Several switching schemes have been in use. However these schemes are cumbersome to operate and are mechanically complex. For a general summary, reference is made to Ligon, W., Multidimensional Gas Chromatography: Techniques and Applications, Chapter 3, pages 55-85 of Gas Chromatography: Biochemical, Biomedical and Clinical Applications, edited by Clement, R., ISBN: 0-471-01048-0, 1990 John Wiley & Sons, Inc., incorporated herein by reference. The present invention addresses the switching problems of the MGCs by using the open-split interfaces of the present invention as a switching system.